Scientists have found what they say could be one of the oldest Stone Age megastructures in Europe: a giant stone wall on the floor of the Baltic Sea. They've dubbed it the "Blinkerwall."

AILSA CHANG, HOST:

Not quite 70 feet below the Baltic Sea, a stunning find has swum into view - a stone wall more than half a mile long that dates back to the Stone Age. It's one of the oldest so-called megastructures on Earth. Science reporter Ari Daniel explains what it might have been used for.

ARI DANIEL, BYLINE: Finding that wall was an accident. It was 2021, and marine geologist Jacob Geersen was teaching a field course at the University of Kiel in northern Germany, a course conducted entirely aboard a research vessel on the Baltic Sea.

JACOB GEERSEN: During night, we were mapping the shape of the seafloor at highest resolution.

DANIEL: One night off the German coast, the students fired up the echo sounders and mapped a swath of seafloor.

GEERSEN: Then when we were sitting together, we saw that there was something special.

DANIEL: It was a ridge that ran for six-tenths of a mile. A year later, Geersen, his colleagues and a new batch of students lowered a camera down and confirmed this ridge was actually thousands of rocks lined up that formed a kind of wall, standing about 1.5 feet tall on average.

GEERSEN: It's usually small stones, but then at some places where we have a large stone, the direction of the wall changes.

DANIEL: Geersen didn't know how such a structure could have formed.

GEERSEN: It was only when we ran to the archaeologists that they said, you may have found something very significant.

BERIT ERIKSEN: I was probably the most skeptical of the entire team.

DANIEL: Berit Eriksen is a prehistoric archaeologist at the University of Kiel. When she examined this structure, a line from Sherlock Holmes came to mind.

ERIKSEN: If you have eliminated all which is impossible, then whatever remains must be the truth. Archaeologists never speak of truth, but I'm running out of things to eliminate in terms of natural stuff.

DANIEL: Eriksen reviewed the data and became convinced the structure was made by prehistoric humans, who'd used lots of smaller stones to connect the larger, unmovable rocks into a wall. She and others concluded it was used by hunter-gatherers some 10 to 11,000 years ago, during the Stone Age, to help them corral and hunt reindeer by the hundreds.

ERIKSEN: The only way you can kill this amount of reindeer is if you cut them off at a pass somewhere. There would have been water at the other side.

DANIEL: So the reindeer would have gotten trapped between the wall and the water, allowing hunters in wait to fire their arrows at the reindeer.

ERIKSEN: If you build a structure like that, you're someone who knows the entire area extremely well.

DANIEL: Ultimately, the area flooded, forming the Baltic Sea we know today and submerging this piece of hunting architecture under water. The findings are published in the journal PNAS.

ASHLEY LEMKE: I know this personally. Working underwater is not easy.

DANIEL: Ashley Lemke is an underwater archaeologist at the University of Wisconsin, Milwaukee who's discovered similar stone walls in Lake Huron beside Michigan. She wasn't involved in this study. Lemke says these structures highlight that people of the Stone Age may have been more sophisticated than we tend to think.

LEMKE: This is actually really early examples of almost animal domestication, right? Like, before you start keeping animals in pens permanently, you're kind of making fences to hunt them.

DANIEL: Leading, maybe, to livestock herding. But to confirm this wall was made by humans to hunt, more evidence is needed, like arrowheads and ancient DNA, in an effort to unite the biggest and smallest clues left behind by prehistoric people.

MARY LOUISE KELLY, HOST:

Ari Daniel, for NPR News.

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